576 C. Robinson and H. A. T. Mills.

positive test for the presence of ramifying aggregates,” although unsupported by direct experimental evidence at the time when it was published, appears to hold good for suspensions of a lyophobic character. The variation with strain of the tensile modulus of gelatin or cellulose acetate jellies finds, according to Poole,* a simple explanation in the presence of a similar branched fibrillar structure ; but whether, or to what extent, solvation contributes to variable viscosity in solutions of lyophilic substances must remain a question of opinion until new experimental methods are devised. At present one generalisation seems to be permissible : if rigid particles suspended in a liquid in which they are insoluble are not prevented from cohering—whether by an electric charge or by an envelope of a soluble substance—they will in time form aggregates, the presence of which will always cause the viscosity to be a function of the rate of shear ; and which, if they are completely interlinked, will also impart rigidity to the suspension as a whole.

The Colloid Chemistry of Dyes: the Aqueous Solutions of Benzo- purpurine 4B and its Isomer prepared from m-Tolidine.—Part I.

By Conm ar R o b in so n and H aro ld A. T. Mil l s , University College, London, and Imperial Chemical Industries.

(Communicated by F. G. Donnan, F.R.S.—Received February 19, 1931.)

Introduction.Substantive cotton dyes may be prepared from such para diamines as

benzidine, diamidoazobenzene, diamidostilbene, etc., and also from homologues of these compounds containing the substituting group in the ortho position to the amido group. I t is a well-known fact, however, that if the substituting group is in the meta position to the amido group the compound does not yield azo dyes which are substantive to cotton, f Thus, although substantive dyes may be prepared from o-tolidine,

they cannot be prepared from m-tolidine.* Loc. cit.t “ The Synthetic Dyestuffs and Intermediate Products,” J. C. Cain and J. F. Thorpe

(1923), p. 58, et seq.

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Colloid Chemistry Dyes. 577

ch3 ch3

So that while we have the well-known substantive dye benzopurpurine 4B, which is prepared from o-tolidine and sodium naphthionate,

we find that the corresponding substance prepared from m-tolidine

S03Na S03Na

has not sufficient affinity for cotton for it to be of practical use as a dyestuff.No obvious explanation of this marked change in properties with the change

of the position of the methyl groups can be put forward on organic chemical lines. As the dyeing of cotton with substantive dyes is generally considered to be a process in which colloidal phenomena play a large part—if not a pre­dominant one—it was thought that a thorough investigation of the physico­chemical and colloidal properties of such a pair of azo compounds might throw some light on this problem as well as on the more general problem of the mechanism of dyeing with substantive cotton dyes and the determination of the factors which decide the dyeing properties of a compound.

The research which is described in this report was therefore carried out on benzopurpurine 4B and the corresponding isomer prepared from m-tolidine (for convenience these two substances will be referred to subsequently as the “ 4B ” dye and the “ meta ” dye respectively). The field of work was at first intentionally limited to only two substances and, further, to a study of their properties in solution (as distinct from their dyeing properties). It is

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578 C. Robinson and H. A. T. Mills.

intended later to extend the work to a study of other dyes, as the results here obtained might suggest, and to a study of the mechanism of adsorption of the dyes on cotton and other substances and the factors which control it. The object of the work here described was therefore a complete investigation of the solutions of these two substances differing greatly in dyeing properties but only slightly in chemical constitution, and the development of methods suitable for this investigation.

Purification.

In general the cotton substantive dyes cannot be recrystallised from water.* Their purification is therefore not easy, and many investigators who have studied the colloid properties of these dyes have worked with impure specimens, with the consequence that their results, as well as being impossible to reproduce, are often quite misleading. The samples of the two dyes with which we worked were specially prepared by the British Dyestuffs Corporation, Ltd., from pure intermediates, the products so obtained containing considerable quantities of electrolytes (sodium chloride and probably sodium naphthionate with traces of sodium acetate and sodium carbonate). Their further purification (freedom from electrolytes) was undertaken in this laboratory.

Several methods have been used for the purification of benzidine dyes, to most of which there are objections. Dialysis gives rise to the formation of the acid dye as Donnan and Harris have pointed out.f Dialysis with sodium hydroxide as the outside liquid (to prevent membrane hydrolysis) will give a product containing alkali—further the method is very slow and, owing to the low solubility of benzopurpurine 4B, unsuitable for preparing large quantities.

Azuma and KameyamaJ salted out congo red with ammonium carbonate and then volatilised the ammonium carbonate. We found that this method gave the ammonium salt of the dye, and their statement that they obtained a sample which gave no reaction with Nessler’s solution must have been due to an error. A sample of congo red treated in the way they describe was obtained chloride-free, but on analysis was shown to contain 96 per cent, ammonium and 4 per cent, sodium, as a percentage of the total alkali radicles present.

* Pelet-Jolivet in his book “ Die Theoriedes Farbeprozesses ” states, without comment, that he purified benzopurpurine 4B by recrystallisation from water. It can only be assumed that he did not actually obtain crystals. The cooling of a 2 per cent, benzo­purpurine 4B solution gives rise to the separation of a tactosol (‘ Zocher, Kolloidchem. Beihefte,’ vol. 28, p. 167 (1929)).

t ‘ J. Chem. Soc.,’ vol. 99, p. 1554 (1911).X ‘ Phil. Mag.,’ vol. 50, p. 1264 (1925).

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Colloid Chemistry of Dyes. 579

Overbeck,* in Zsigmondy’s laboratory, purified congo red by continued washing in the pressure ultrafilter (this is fitted with an electromagnetic stirrer and pressures up to 125 atmospheres can be applied) until the conductivity of the ultrafiltrate was of the same order as that of the distilled water used. We tried this method, using the same apparatus, but found that with benzo- purpurine 4B the filtration, even when using the electromagnetic stirrer, was so extremely slow that the method was not practical for preparing the quantities of this dye required.

Several workers have used the method of precipitating the acid dye and neutralising it with the theoretical amount of sodium hydroxide. This method does not seem very reliable as, firstly, there is a difficulty in preparing the pure acid dye (prolonged washing peptises it) and, secondly, the acid dye being insoluble it is doubtful if the equivalent quantity of caustic soda would neutralise all of it, so that the final product might be expected to contain occluded acid dye.

The method finally decided on was repeated, salting out with sodium acetate and washing out the sodium acetate with absolute alcohol (potassium acetate, although more soluble in alcohol, cannot be used if the sodium salt of the dye is required). The sodium acetate used was British Drug Houses’ “ A.R.” product, recrystallised. A 40 per cent, solution of this was made up. 40 grams of the impure dye was stirred up with 75 c.c. of distilled water and heated until boiling with constant stirring. 75 c.c. of the sodium acetate was now added and the whole allowed to stand. The dye was then filtered off bv suction on a Buchner funnel, a colourless filtrate being obtained. The dye was again stirred up with water and the salting out process continued. This procedure was carried out five times, the filtrate and the dye itself were then found to be free from chloride.

The dye was now boiled with alcohol, transferred to the Buchner and washed two or three times with hot alcohol. The boiling and washing with alcohol was carried out five or six times. The dye, after this, no longer gave the cacodyl test for acetate. I t was then dried at 105° C. Care was taken not to let the temperature go above this, as at a temperature of about 120° C. there was some decomposition accompanied by a darkening of the dye. All vessels used during the purification were of Jena or Pyrex glass.

The sodium content of the two dyes was determined, the following

* W. Overbeck, “ Verbesserungen an Methoden zum Untersuchung von kolloiden Farbstoffen ” (* Dias,’ Gottingen, 1926),

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580 C. Robinson and H. A. T. Mills.

results being obtained. This determination was made as accurately as possible.

i

Sodium found. Sodium theoretical.Percentage pure

dye calculated on sodium found.

Meta dve ..........................per cent.

6*32per cent.

6-34 99-7 100 04B d ye............................... 6*34 6-34

The sodium content of both dyes agreed with the calculated values within the limits of the experimental error of the method of determination.

Titration of a weighed quantity of the purified dyes gave with titanous chloride in the presence of sodium citrate 101 per cent. This high result is probably due to a methodical error, accurate absolute values being difficult to obtain owing to the precipitation of the dye before reduction is complete.

Preparation of Solutions.

As the dyes were found to be hygroscopic, solutions were made up by weigh­ing off approximately the amount of the dye required and dissolving in the necessary amount of distilled water, boiling for 1 minute, to ensure complete solution, and filtering through an ash-free filter paper. The exact concentra­tion of the dye was determined by evaporating 25 c.c. of the solution in a wide­mouthed weighing bottle on a water bath until the residue of dye just began to flake off the bottom, the weighing bottle then being transferred to an oven and heated at 105° C. until its weight remained constant.

All solutions were made up and preserved in pyrex flasks fitted with ground- glass stoppers.

Viscosity and Ageing of Solutions.The viscosity of benzopurpurine 4B is said to depend on the method of

preparation and the age of the solution. Thus Ostwald* found that the viscosity of a 0 • 3 per cent, benzopurpurine solution prepared cold was from 1-041 to 1-075, depending on the pressure applied to the capillary viscometer, while dissolved hot the viscosity was from 1-239 to 1-723. He does not mention purifying the dye. Similarly Hatschekf found the modulus of elasticity and the viscosity (measured in the Couette viscometer) dependent on the history of solution, a 1 per cent, solution having a low viscosity if

* ‘ Z. Phys. Chem.,’ vol. I l l , p. 62 (1924). t ‘ Kolloid Z.,’ vo]. 39, p. 300 (1926).

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Colloid Chemistry of Dyes. 581

prepared cold, and 100 times that of water if prepared hot. LiepatoS,* using the Ostwald viscometer, found for 0-5 per cent, solution a relative viscosity of 1-68 at 18° C.

As the dependence of any property of the dye solution on either the method of preparation or the age of the solution would have to be taken into account in the study of these dyes, we carried out some measurements with an Ostwald viscometer. Bungenberg de Jongf has fully described the conditions under which an Ostwald viscometer should be used so as to obtain reliable results with an accuracy of 0 • 1 to 0*2 per cent. The viscometer we used was there­fore constructed (out of durosil) in accordance with these principles. The diameter of the capillary was 0 • 34 mm. and the length of the capillary was so chosen as to conform with the formula of Griineisen.J

The ends of the capillary opened out gradually into the wider parts of the tube, this reducing the tendency to form eddy currents. The time of flow with 10 c.c. of water in the viscometer was 95-4 seconds. The stand recommended by de Jong was not found necessary, as no difficulty was found in obtaining readings checking within 1/5 second for several different settings up of the viscometer in the thermostat when using an ordinary clamp and plumb line. The measurements were made at 25° C., the temperature at no time being allowed to vary more than 1/40° C. The influence of ageing is shown in the following measurements made on solutions of the respective dyes prepared hot by the method described.

Table I.

Meta dye. 4B dye.

Concentration (grams per 100 c.c.) ....................... 0*569 0-5717] at 25° after 1 day ..................................... ........... 1*035 1-035

,, 12 days ................................................ 1*036 1-036,, 6 weeks ............................................ 1-037,, 6 months............................................ 1*040 1-041

We also made up two solutions of the 4B dye of the same concentration (0*57 grams per 100 c.c.), one of which was prepared cold, and the other hot, and measured their viscosities. The viscosity of the one prepared cold, which

* ‘ Kolloid Z.,’ vol. 39, p. 230 (1926).f ‘ Rec. Trav. Chim., Pay-Bas.,’ vol. 42, p. 1 (1923).t * Wiss. Abh. Phy. Tech. R. Anst.,’ Berlin, vol. 4, p. 151 (1905).

VOL. CXXXI.—A. 2 R

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582 C. Robinson and H. A. T. Mills.

had to be shaken vigorously to obtain solution, was measured after 20 hours, while the one prepared hot was measured after 3 hours. The results were :—

Prepared hot. Prepared cold.1-034 1-034

Freundlich and Shalek* using the Hess viscometer and also the Couette viscometer, showed that the viscosity of a benzopurpurine solution varied with the rate of shear, i.e., the solution did not obey Poiseuille’s law. Ostwaldf showed that similar results could be obtained with the Ostwald viscometer if varying pressure, positive or negative, were applied to the capillary of the viscometer. We therefore carried out some experiments to see if the viscosity of our dye solution was also dependent on the rate of flow through the capillary. Pressure, positive or negative, was applied to the Ostwald viscometer by means of an aspirator fitted with a water manometer. The viscometer was first filled with water and the rate of flow measured for different readings of the manometer varying from —3 to +10 cm. of water. From the results a curve was plotted and subsequently when the rate of flow for a dye solution at a certain pressure was measured, the corresponding rate of flow for water could be read from this curve. The results are given as the ratio of the time of flow for the dye (TD) to the time for water (Tw), this differing by less than the experi­mental error from the relative viscosity of the dye (the density of the dye solution being only 1-002). I t will be seen that within the range of pressure used the viscosity does not vary by more than 0-005. The experimental error was rather greater than with the Ostwald viscometer as ordinarily used, but the results were reproducible to within about 0-004.

Table II.—Viscosity of a 0-5 per cent. “ Meta ” Dye Solution at various Ratesof Flow (at 25° C.).

Tr. Tw- Td/Tw

seconds seconds145-6 141-0 1-033121-4 117-0 1-038108-4 104-5 1-03899-1 95-5 1-03863-4 61-2 1-03555-4 53-4 1-03852-4 50-6 1-036

* ‘ Z. Phys. Chem.,’ vol. 108, p. 152 (1924). t ‘ Z. Phys. Chem.,’ vol. I l l , p. 62 (1924).

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Colloid Chemistry of Dyes. 583

Table III.—Viscosity of a 0-5 per cent. “ 4B ” Dye Solution at various Ratesof Flow (at 25° C.).

t d. Tw Td/Tw

seconds seconds138-6 134-5 1-032136-2 131-3 1-035133-2 128-5 1 03695-7 98-8 1-03257-2 55-2 1-03651-8 50-2 1-031

We may therefore conclude from these experiments that viscosities of these two dyes are the same within the experimental error and are of the same order as that of a typically lyophobic solution (i.e., they do not greatly exceed the viscosity of water). The particles are therefore either unhydrated or only slightly hydrated. Also the viscosity is independent of the method of prepara­tion and age of the solution. Further, the viscosity obeys Poiseuille’s law and does not vary with the rate of shear—in other words, there is no structural viscosity. The much higher and varying viscosities obtained for benzo- purpurine 4B by the other workers were due to either the solutions with which they worked being in a state of incipient coagulation due to the presence of electrolytes, or due to the concentration of the dye being above the saturation point, or both these causes.

Flocculation with Electrolytes.In determining the flocculation values of a solution for a certain electrolyte,

it is usual to determine the minimum concentration of electrolytes which will bring about complete flocculation in a given time. In the case of benzopur- purine 4B this was quite possible to determine as there was a fairly sharp point where complete flocculation was obtained, giving a bulky gelatinous precipitate of the dye with a quite clear colourless liquid above this. Thus, when two portions of sodium chloride solution were added to one portion of a 0-5 per cent, benzopurpurine 4B solution, this was brought about when the final concentration was 0-16 moles, of sodium chloride per litre. The meta dye, however, behaved differently, even when under the same conditions, the final concentration of sodium chloride was 2-30 moles., complete flocculation was not brought about; the liquid above the precipitate being still coloured (it should be noted that a very small quantity of dye is necessary to give a con-

2 r 2

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584 C. Robinson and H. A. T. Mills.

siderable colour, as one part of dye in 10 million of water can be easily detected). The precipitate for the meta dye was not gelatinous like that of the benzo- purpurine 4B and was far less bulky. Since for our purpose it was more important to obtain results for the two dyes which could be compared, we determined the minimum concentration of electrolyte which would cause the solution to become cloudy.

Experimental.

The experiments were carried out in flat-bottomed pyrex test-tubes, which had previously been carefully cleaned with chromic acid and then steamed out. 2 c.c. of a solution of dye containing 5 grams per litre were placed in a number of these, and 4 c.c. of various concentrations of sodium chloride solution were added and the tube shaken. The large volume of salt solution added is necessary in the case of the meta dye, as here we have to add quantities of sodium chloride which are near the saturation point of that salt. By only adding an equal volume of salt solution the necessary final concentration could not be attained. Care was taken in each case to follow exactly the same procedure for adding the electrolyte and shaking in the tube. After 24 hours the concentration of sodium chloride which made the contents of the tube so cloudy that they could not be seen through was noted, as well as the lower concentration which did not give this cloudiness. A second series of tubes was then set up containing concentrations ranging between these two con­centrations, and finally a third series, so that the limiting value is obtained as accurately as possible. The results are given in Table IV. We also show the results obtained for calcium chloride and aluminium chloride. It is, however, doubtful if these two salts do not introduce other complications as both have an acid reaction. In the case of the aluminium chloride in particular, the colour of the precipitate suggested that some of the insoluble blue acid dye had been formed. Washing the precipitate given by aluminium chloride repeatedly with distilled water, redissolved it however. The results for NaCl were quite sharp and easily reproducible for both dyes. The results are accurate to about 5 per cent. Repeating the experiments 6 weeks later gave the same results within 5 per cent., so that here also, as with the viscosity experiments, there seemed to be no appreciable ageing effect. The results obtained with NaOH were similar to those for NaCl, the flocculation values being somewhat higher.

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Colloid Chemistry of Dyes.

Table IV.—Flocculation by Electrolytes.

585

4B dye. Meta dye.

NaCl....................................................moles./litre.

0 072moles./litre

2-23 0 0630 -004 (about)

'2-60

CaCl2 ................................................ 0 0037A id s .................................................... 0*0004 (about)

0*083NaOH ................................................

The small amount of sodium chloride necessary to bring about the complete precipitation of benzopurpurine 4B would seem to suggest that we were here dealing with the flocculation of a typical lyophobic solution, where flocculation is brought about by the added electrolytes lowering the potential at the surface of the colloidal particle to a critical potential at which coagulation can take place. On the other hand, the high concentration necessary to bring about a precipitate in the case of the meta dye and the fact that this precipitate increases in quantity with increasing quantities of sodium chloride added (up to a point where we are adding a saturated solution of sodium chloride to the dye solu­tion), suggests that the precipitation here is more of the nature of the “ salting out ” of a soluble substance. To throw further light on this we determined the flocculation values for different concentrations of dye. If the precipitate was merely a “salting out,” or lowering of the solubility of a substance it would be expected that on diluting the dye considerably more electrolyte would be needed to bring about precipitation, while in the case of a true colloidal flocculation, the increase of the flocculation value on dilution would not be so great and there might in fact be a decrease* (as in the case for BaCl2 and A1C13 with the As2S3 solution). Our results were as follows :—

Table Y.—Variation of Flocculation Values with Concentration of Dye.

Concentrations.

NaCl (moles, per litre).

4B. Meta.

20*0 grm. per litre ................................... . 1-776-0 „ .................................... 0-072 2-231-25 „ ................................... 0 090 2-53

* Kruyt and van der Spek, ‘ Koll. Z.,’ vol. 25, p. 1 (1919); also H. R. Kruyt, Jerome Alexander, “ Colloid Chemistry,” vol. 1, p. 306.

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586 C. Robinson and H. A. T. Mills.

These results then point to colloidal flocculation, especially in the case of the 4B dye. It is most likely that both factors (lowering of potential and “ salting out ”) play a part in each of these dyes, but that in the case of benzopurpurine 4B the colloidal precipitation predominates. These results then suggest that in the “ 4B ” dye solution the particles are big enough for the charge at the surface of the particle to be of significance while the “ meta ” is either in true solution or more nearly in true solution. For comparison we give the floccula- tion values for sodium chloride for four dyes obtained by Frl. Beger* (the method of determining the end point is not stated).

Table VI.

Concentration of dye Flocculation value for NaCl.

Congo rubin ............................................grams per litre

1 0millimoles, per litre

105Benzopurpurine 4B.................................... 1 0 105Benzopurpurine 10B ................................ 1-0 480Congo red ............................................ . 1-0 980

From these results it would seem that benzopurpurine 10B and congo red are cases intermediate between the “ 4B ” and “ meta ” dyes.

Ultramicroscopic Examination and Streaming Double Refraction.It is generally stated in the literature that solutions of benzopurpurine 4B

when looked at in the ultramicroscope are found to contain ultramicrons which are non-spherical. Fraulein Begerf says that although a 0-01 per cent, solution of benzopurpurine 4B is almost optically empty, ageing phenomena can be plainly observed and after 3 months the same solution shows numerous needle-shaped particles.

We found that a solution made up from the dyes that had not been freed from electrolytes gave these ultramicrons, their long shape being plainly seen if a suitable dilution was chosen. A 0 • 5 per cent, solution of the purified dye, however, showed no ultramicrons (that is, the solution only contained a quantity of ultramicrons comparable to that in the distilled water) when examined either undiluted or diluted 50 times. The six-month-old solution showed somewhat more particles, but still very few, no needle-shaped particles

* ‘ Z. Phys. Chem.,’ vol. I l l , p. 227 (1924).t Beger, ‘ Diss.,’ Gottingen (1923); Zsigmondy, ‘ Z. Phys. Chem.,’ vol. I l l , p. 223

(1924).

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Colloid Chemistry of . 587

being observed in either solution. The addition of sodium chloride to the benzopurpurine was found to give ultramicrons as observed by other workers. Thus, if two volumes of sodium chloride solution were added to one volume 0*5 per cent, dye solution so that the final concentration of sodium chloride was 0*05 moles, per litre, this mixture was found to contain masses of ultra­microns which gave the scintillating effect associated with non-spherical particles. The occurrence of needle-shaped particles on ageing, described by other workers, may therefore have been due to impurities in the dye solutions.

All the solutions of benzopurpurine 4B which were shown to contain ultra­microns were also found to give streaming double refraction. This was observed by the “ vortex ” method of Zocher.* The dye to be examined is placed in a flat-bottomed cylindrical tube. A beam of polarised light enters the tube at the bottom, parallel to the axis of the cylinder. The dye is then observed from above using a Nicol prism as analyser. The nicol being crossed so that darkness is obtained, the cylinder is rotated about its axis, whereupon when the dye settles down to a steady motion four light quadrants are seen and a dark maltese cross between the quadrants, this figure disappearing again as soon as the solution has come to rest. This streaming double refraction, as Schuster has pointed out, is also observed at concentrations of electrolytes which are insufficient to give rise to ultramicrons. The method is very sensitive, it being possible to see the double refraction in extremely dilute solutions.

With the “ meta ” dye, however, in no case did we obtain ultramicrons (that is in any considerable quantity), or streaming double refraction, either with small quantities of sodium chloride or with concentrations which were nearly sufficient to bring about flocculation ( ., over 2-00 moles, per litre).When 2 • 00 moles, per litre of sodium chloride were present a certain number of ultramicrons could be seen, but these, unlike those obtained with the “ 4B ” dye, did not scintillate. In no case was there any streaming double refraction observed, this applying also to additions of calcium chloride and aluminium chloride.

The.difference in the behaviour of the two dyes is here very marked.

Solubility.The solubility of benzopurpurine 4B is difficult to determine accurately

owing to the difficulty of separating the undissolved dye from the saturated solution. Thus W. C. Holmesf who determined the solubilities of a number of

* ‘ Z. Phys. Chem.,’ vol. 98, p. 293 (1921). t ‘ Stain Tech.,’ vol. 3, p. 12 (1928).

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588 C. Robinson and H. A. T. Millsa

dyes, found that in the case of benzopurpurine 4B “ extremely colloidal solu­tions ” were formed, and he found no means of separating the dye. For these reasons we made no attempt to obtain an accurate value of the solubility, but only arrived at a method that would bring out any marked difference in the solubilities of the two dyes.

The solubility was determined at room temperature in two ways : A, the dye was shaken for 12 hours with distilled water and then the undissolved dye allowed to settle for days until a clear solution was obtained ; B, a super­saturated solution was kept at room temperature until the precipitate forming from it had settled. The result in the case of the “ 4B ” was only very approxi" mate, owing to the difficulty found in separating the saturated solution.

A. B.Meta d y e ......................... 5*97 grm./lOO c.c. 5*58 grm./lOO c.c.4B d y e ............................. 0*8 „ 0*8 ,,

These results are sufficiently accurate to bring out the marked contrast in the solubilities of these two dyes.

Hydrogen Ion Concentration.The pn of solutions of each of the dyes as well as that of congo red (similarly

purified) were measured electrometrically, using a hydrogen electrode. The following results were obtained for 0*5 per cent, solutions :—

Meta d y e .................................... 7*134B d y e ........................................ 7-10Congo red ................. 7*18

Hydrolysis in these dye solutions may, therefore, be considered negligible. Zsigmondy found the degree of hydrolysis to be negligible for congo red, benzopurpurine 4B and benzopurpurine 10B.

Ultrafiltration.This was carried out in the Zsigmondy* pressure ultrafilter, using the

Zsigmondy “ ultrafeinfilters/’f The apparatus, which is made by Membran- filter G.m.b.H., of Gottingen, is described fully elsewhere.*

With this apparatus it is possible to use pressures up to 125 atmospheres. The electromagnetic stirrer was removed for these and the subsequent con-

* Brukner and Overbeck,’ ‘ Koll. Z.,’ vol. 36, ‘‘ Zsigmondy Festschrift,” p. 192 (1925). t ‘ Z. Anorg. Chem.,’ p. 398 (1926); ‘ Biochem. Z.,’ vol. 171, p. 198 (1926).

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Colloid Chemistry of Dyes. 589

ductivity experiments, as it was found to be superfluous for the dye solutions used, and only added to the difficulty of cleaning the apparatus. All parts of the apparatus with which the solution comes in contact were heavily tinned.

The ultrafeinfilters are made of nitrocellulose, and can be obtained of various pore sizes. The numbers of the filters here given are those given by the makers and refer to the time (in minutes) taken to filter 100 c.c. of water through a filtering surface of 100 sq. cm. under a pressure of 60 to 70 cm. of mercury. Using a 0 • 5 per cent, solution of each dye, we determined the filter of largest pore size which would just prevent the dye particles from passing through. The apparatus was washed very thoroughly with distilled water before use, and, after setting up, several lots of distilled water were filtered through the apparatus, the final ultrafiltrate having a conductivity comparable to that of distilled water.

Table VII.

Filter.

Appearance of filtrate.

Meta dye. 4B dye.

min. sec. 16 0 Colourless Colourless10 0 Distinctly coloured 99

7 0 Coloured 99 *

3 0 Passes through unchanged 9 9

2 0 99 99

1 25 99 Passes through unchanged

The results were reproducible. They also were independent of the order in which the dye solutions were filtered, so that the results were not due to dye particles adsorbed in the pores of the filter preventing the passage of the “ 4B ” dye. The method of graduation of these filters gives, of course, only the mean pore size of the filter. Even, however, if there is some doubt about the exact pore size of the filters here used, there seems no doubt that there is a very big qualitative difference in the ultrafilterability of these two dyes.

Conductivity.The conductivity of a dye solution is the sum of the conductivity of the dye

particles and the conductivity of the intermicellar liquid. For the purpose of this research we are more interested in the conductivity of the dye particles, and to find the value of this it is necessary to know the conductivity of the

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590 C. Robinson and H. A. T. Mills.

intermicellar liquid. This can be obtained by ultrafiltering the dye and measuring the conductivity of the ultrafiltrate, a procedure which was followed by Willy Oberbeck* in studying the conductivity and mobility of congo red. For this purpose then we used the Zsigmondy pressure ultrafilter and ultra- feinfilters described in the last section. As the determination of the con­ductivity of the dye particles is obtained by a different method, it is necessary, especially when studying the more dilute dye solutions, to have a conductivity apparatus with which highly accurate measurements can be made and which will be suitable for the measurement of the conductivity of very dilute solutions.

Apparatus.The conductivities of the solutions were measured by means of a Wheat­

stone bridge. The source of alternating current was an oscillating circuit giving a current of about 1000 cycles, to which frequency the ear is most sensi­tive. The metre scale was graduated in millimetres and fitted with a vernier, so that readings correct to a tenth of a millimetre could be made. The wire was carefully standardised before use. The conductivity cell was an ordinary Kohlrausch cell, made of Jena glass. The variable resistance in the other arm was a non-inductively wound four-dial resistance box, which had been standardised by the National Physical Laboratory. A variable capacity was put in parallel with the resistance box and “ tuned ” until silence was obtained in the telephones at the null point.

All connecting wires were of lead shielded cable. The resistance box, metre scale, etc., were laid out as symmetrically as possible and placed on sheet iron. The oscillating circuit was shielded by being placed in a copper box. This copper box, the leaden shielding of the wire, the copper of the thermostat, and all other shieldings, were connected together by soldered wires and earthed. This apparatus was found to be very sensitive and the null point could be determined correct to 0*1 mm. For determining the null point, a pair of 2000 co ear-phones were used. The thermostat was kept at 25° C. and did not vary by more than 0 • 005° C. throughout the experiments.

The Solutions.An approximately 0*5 gram per 100 c.c. solution of the dye was made as

described already, and the exact concentration determined by evaporating 25 c.c. to dryness. The more dilute solutions were made up from this by successive dilution. The distilled water was used (sp. conductivity, 3*6 X 10~6 reciprocal ohms) as it came from the still. No attempt was made

* ‘ Diss.,’ Gottingen (1926).

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Colloid Chemistry of Dyes. 591

to use CO 2 free water, as the conductivity of the water when passed through the ultrafilter became considerably higher than the ordinary water and the use of the ordinary distilled water reproduced more closely the conditions of the solutions used for other experiments, where they came in contact with the atmosphere.

To obtain a sample of the ultrafiltrate, a 16-minute “ ultrafeinfilter ” was placed in the pressure ultrafilter and successive quantities of distilled water filtered through until the conductivity of the water which had passed through the filter remained constant and was of the same order as the conductivity of the water before filtering. Very prolonged washing was sometimes necessary for this. 30 c.c. of the dye solution was then placed in the filter and the conductivity of the ultrafiltrate was measured, rejecting, of course, the first few cubic centimetres that passed through. A second sample of the dye was then filtered and another measurement made, the mean of these two values being taken. All the filtrates were quite colourless.

The results obtained are shown in Tables VIII and IX. The specific con­ductivity of the intermicellar liquid was obtained by subtracting from the •observed conductivity of the ultrafiltrate a correction factor C, which was the

Table VIII.—Conductivities of Meta Dye Solutions.

Solution number.

1. 2. 3. 4. 6. 8. 10.

KWI x 106 ........................... 3-62 3-59 3-62 3-62 3-62KWII X 106 ....................... 6-84 — — 4-49 4-81 4-45 4-02C X 10« ................................ 3-22 _ — 0-90 1-19 0-83 0-40Ks X 106 ........................... 1017-4 593-9 317-1 169-85 46-394 14-61 6-872Ku X 10« ........................... 35-85 — — 10-19 6-58 5-22 4-66K i x 106 ........................... 32-63 (21-0) (13-8) 9-29 5-39 4-39 4-26Km x 106 ........................... 984-8 572-9 303-28 160-56 41-0 10-22 2-61T) x 104 ............................. 145-7 72-85 36-43 18-22 4-554 1-136 0-284V io o o v ............................... 2-441 1-938 1-538 1-222 0-7693 0-4844 0-3051A ........................................... 67-60 78-5 83-24 88-10 89-03 89-97 91-89

Kwl = Specific conductivity of the water before filtration.KwII = Specific conductivity of the water after filtration.Ks = Specific conductivity of the dye solution.Ku = Specific conductivity of the ultrafiltrate.KwII — Kwl = C = Correction factor.K* = Kw — C = Specific conductivity of intermicellar liquid.Km = Ks — K$ = Specific conductivity of the dye.V = Equivalent concentration.A = Equivalent conductivity.The figures for K* for solutions Nos. 2 and 3 were obtained by interpolation.

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592 C. Robinson and H. A. T. Mills.

Table IX.—Conductivities of “ 4B ” Dye Solutions.

Solution numbers.

1. 2. 3. 5. 7. 9.

KWI X 106 ................... 3-21 3-21 3-08 3-31 3-21 3-21KWII X 106 ................... 4 1 5 4-90 6-11 3-84 4-56 4-11C x 106 ............................... 0-94 1-69 3-03 0-63 1-35 0-90Ks X 10« ....................... 956-5 521-5 293-8 85-64 27-61 12-68K X 106 ....................... 23-3 14-88 13-18 11-45 8-88 8-62K i X 10« ....................... 22-4 13-19 10-15 10-82 7-33 7-72K m X 106 ....................... 934-1 508-3 283-7 74-79 20-28 4-96r) X 104 ........................... 133-84 66-92 33-46 8-365 2-092 0-5228\rm o ~ v ....................... 2-313 1-885 1-496 0-9420 0-5933 0-3739A .................................... 69-79 75-91 84-82 89-39 96-96 94-84

amount that the conductivity of the distilled water increased by on filtering through the apparatus. The specific conductivity of the intermicellar liquid was then subtracted from that of the dye solution, this giving a value which represents the specific conductivity due to the dye itself. From this value we obtain the equivalent conductivity of the dye A, the equivalent concentration being taken as twice the molecular concentration. The equivalent con­ductivity of the dye determined in this way we find approaches a limiting value with progressive dilution, while the equivalent conductivity of the dye solution increases with increasing dilution.

In the figure we show the equivalent conductivity of both dyes plotted against \ / 1000 7], where r\ is the equivalent concentration. For the “ meta ” the

curve runs almost horizontal for the more dilute solution and we obtain by extrapolation a value of 92 reciprocal ohms for the equivalent conductivity

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Colloid Chemistry of Dyes. 593

at infinite dilution, A#,. The conductivity of the 4B dye (broken line) does not differ by more than 1 or 2 per cent, from that of the meta dye in the more concentrated solutions. The curve, however, is not so regular as for the meta dye and the value for A» obtained by extrapolation is therefore more doubtf ul.

This is partly because the conductivity of the intermicellar liquid for the more dilute solutions was greater for the meta dye than in the case of the 4B dye, which renders the values for the last two solutions less reliable and hence throws doubt on the final direction of the curve. By extrapolation we obtain a value of 98 reciprocal ohms for A*. There seems, however, to be no great difference between the conductivity concentration curves of these two dyes. The curve for benzopurpurine 4B agrees very closely with the curve obtained by Frl. Beger, which is reproduced by Zsigmondy* except that for the very dilute solutions i f / 1000 rt < O’ 1) Frl. Beger’s curves show a much more rapid increase of equivalent conductivity with dilution. By extrapolation they obtain Ax = 105.

From the similarity of the values for A* we must conclude that the values for the mobilities of the anion cannot greatly differ. This, however, does not necessarily mean that the anions are of the same size. Thus McBainf has found in his researches on the soaps that a more complex anion may actually have a greater mobility than the sum of the mobilities due to the simpler anions which constitute it—the more complex ion, containing the same number of charges as the sum of the constituent ions offering less resistance to the motion than do the single ions.

The value of the mobility of the anion (u) for a 0 • 1 per cent, benzopurpurine 4B solution has been determined by Frl. BegerJ with Galecki’s§ modification of the Coehn U-tube and found to be 47*9. The value of v being 50*9 this would mean that the mobility of the anion does not alter greatly between this concentration and infinite dilution since we have obtained a value of 92 forA qu .

In calculating the equivalent concentration for obtaining values for the equivalent conductivity, we have taken the equivalent as being half the molecular weight of the dyestuff. The “ electroequivalent ” (by which term Zsigmondy11 denotes the number of molecules associated with one electric

* ‘ Z. Phys. Chem.,’ vol. I l l , p. 216 (1924).t ‘ Z. Phys. Chem.,’ vol. 76, p. 179 (1911).t ‘ Diss.,’ Gottingen (1923).§ ‘ Z. Anorg. Chem.,’ vol. 74, p. 174 (1912); c/. Zsigmondy, “ Kolloid Chemie,” 3rd ed.

p. 61.|| “ Kolloidchemie,” 3rd ed., p. 173.

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594 C. Robinson and H. A. T. Mills.

charge) will, however, not be equal to the chemical equivalent if all the sodium of the dyestuff is not dissociated. For this reason W. Overbeck, in the disserta­tion already referred to, attempts to calculate the electro-equivalent of congo red by the method of Wintgen* using the formula

. _ G + ve ~ M ‘ 1000 Km’

where Ae = electro-equivalent, G = number of grams of the colloid per litre, M = simplest molecular weight of the dissolved substance, u -f- v the sum of the mobilities, Km = specific conductivity of the micelle; and then calcu­lates the equivalent conductivity using this value. He obtained a value for Ae in the higher concentrations, determining u by the U-tube method at this concentration. For the more dilute solutions it was not possible to determine u, and he assumed that Ae did not change greatly with dilution. His evidence for this assumption being that with Bordeaux extra, while it was found possible to determine u for a large range of concentrations, A did not vary by much. To assume that other dyes behave as Bordeaux extra seems to us, however, not justified, Bordeaux extra, as we shall point out later (Part II) being probably much nearer to a condition of true solution than either congo red or benzo- purpurine 4B.

We give, however, as a matter of interest the value for Ae calculated from Frl. Beger’s result for u already referred to :—

G. M. u . V. u + V. 1000

1 0 0 726-1 47-9 50-9 99-8 0-240 0-57

Summary.A study has been made of the solutions of benzopurpurine 4B and the

isomer prepared from meta tolidine.(1) A method has been devised for the purification of these and similar

dyestuffs.(2) I t has been shown that the phenomenon of ageing which takes place in

impure solutions of benzopurpurine 4B does not appear in either of these dye solutions when pure.

(3) The viscosities of solutions of these dyes (if not supersaturated) are the

* Wintgen, ‘ Z. Phys. Chem.,’ vol. 103, p. 254 (1922).

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Colloid Chemistry of Byes. 595

same and are of the order to be expected in an unhydrated (lyophobic) colloid. The viscosity does not vary with the rate of shear.

(4) pu determinations have shown hydrolysis to be negligible.(5) The “ solubility ” of the “ meta ” dyes is much greater than the “ 4B ”

dye.(6) Flocculation experiments with electrolytes point to the particle size

of the “ 4B ” dye being of colloidal dimensions, while that of the “ meta ” is more nearly in true solution.

(7) Ultrafiltration experiments with graduated ultrafilters show that the particle size of the “ 4B ” is larger than the “ meta.”

(8) While solutions of the pure dyes show no particles in the ultramicroscope, non-spherical ultramicrons are obtained by the addition of small quantities of electrolyte to the “ 4B.” In the meta dye ultramicrons could not be produced.

(9) Streaming double refraction which can be observed in solutions of the “ 4B ” to which electrolytes had been added was in no case observed in the “ meta.”

(10) Conductivity measurements showed the conductivities of the two dyes to be almost the same. The conductivities were measured over a large range of concentrations. By subtracting the conductivity of the ultrafiltrate, that of the “ micelle ” was obtained. The similarity of the results for the two dyes is, however, not evidence that the (multiple) anions are of the same degree of complexity.

I t will be seen, therefore, that there is a marked difference in the properties of the solutions of these two dyestuffs, which could be explained on the assump­tion that the 4B dye forms larger aggregates than the meta dye. This will be further discussed in Part II.

The authors desire to express their best thanks to Professor F. G. Donnan, C.B.E., F.R.S., for his very kind interest in this work.

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